High-Temperature Testing of Stanyl Plastic Gears: A ...

1 March/April 2010 GEARTECHNOLOGY 59 Management Summary This paper shows an experimental study on the fatigue lifetime of high -heat polyamide ( Stanyl ) gears running in oil at 140 C. Based on previous works (Refs. 1 2), an analysis is made correcting for tooth bending and calculating actual root stresses. A comparison with tensile bar fatigue data for the same materials at 140 C shows that a good correlation exists between gear fatigue data and tensile bar fatigue data. This insight provides a solid basis for gear designers to design Plastic gears using actual material 1 (a) load sharing and (b) root stresses as function of roll angle for a steel pinion and a steel gear (black circles), a glass-fiber (GF) reinforced- Plastic gear (red circles), an unfilled Plastic gear (blue circles) and an unfilled Plastic gear at elevated temperature (green circles).

2 The dashed black line is the load sharing according to (steel) theory (ISO 6336) and the dotted black line repre-sents the pitch materials have been used in gearing for quite some time. Over the last decade the application field for Plastic gears has extended from only low loads, positioning type of transmis-sions, to increasingly more demanding applications with high loads, high num-bers of cycles and high temperatures . This implies that during the design pro-cess not only the quality of the geom-etry is important, but also the dimen-sioning with respect to stresses. The standards (ISO, DIN, AGMA) which are currently used by gear designers have a proven track record for metal gears; however, they certainly lack features that are of importance for Plastic gears.

3 In addition to this, the experimental data on gears available for today s gear materials are rather a previous study, it was shown that the kinematics and stress distribu-tion in a metal- Plastic gear pair can be quite different from a metal-metal gear pair (Refs. 1 2). The main reason for this is the fact that the stiffness-strength ratio of plastics is lower, com-pared to steel. As a result, the deforma-tion and tooth bending under loading are far more pronounced for Plastic High-Temperature Testing of Stanyl Plastic Gears: A Comparison with tensile Fatigue DataDr. Ir. HGH van Melick and Dr. HK van Dijk(Printed with permission of DSM Engineering Plastics)Figure 1. (a) load sharing and (b) root stresses as function of roll angle for a steel pinion and asteel gear (black circles), a glass fib re (G F) reinforced Plastic gear (red cir cles), an unfilled plasticgear (blue ci rcles) and an unfilled Plastic gear at elevated temperature (green ci rcles).

4 Thedashed black line is the load sharing ac cording to (steel) theory (ISO 6336) and the do tted blackline represents the pi tch Due to this load sharing over tooth pairs in contact (Fig. 1), the con-tact path and contact ratio are consider-ably 1b shows that the changes in load sharing influence the root stresses of the gear pair. It is not the modulus that directly affects the stresses, but the changes in contact ratio and tooth bending that via load sharing influ-ence the root Figure 1a it is also observed that by going from a steel-steel pairing to a steel-glass-filled Plastic pairing, the period of single tooth contact is halved. For the unfilled Plastic case and the unfilled Plastic at elevated tem-perature case, single tooth contact no longer occurs during meshing.

5 With decreasing modulus the maximum load share decreases to a plateau value of approximately 2/3. This increase in contact ratio was shown to result in a substantial decrease in root stresses (Table 1). The question now arises whether the root stresses can be related to mate-rial properties. For metallic materials, the correlation between material prop-erties ( tensile strength, fatigue strength, etc.) and the actual performance of a gear are quite well established. This is not the case for polymers. Along with GEARTECHNOLOGY March/April 2010 in kinematics and stress-es, the effect of Testing conditions like temperature, strain rate, humidity, etc., play a huge role in their addition, an increase in contact path length is observed in Figure 1 by going from the steel-steel mesh to the steel- Plastic material at elevated T-mesh.

6 The combination of increasing contact path length and tooth bending was also shown to have a big influ-ence on the contact stresses in the same work (Refs. 1 2). The contact stress picture is shown in Figure expected based on contact mechanics and normalized calcula-tions the contact stresses near the pitch point where, according to clas-shown to exist (Refs. 3 4).Intent of this StudyDesigning gears requires a degree of experience. Many new designs are based on proven concepts of the past. This is certainly true for Plastic gears, where at the moment this compara-tive, best practice method is the saf-est way to operate. However, bottom line is that, in principle, a gear tooth is an odd-shaped bending beam, and the expected lifetime of this bending beam under fatigue loading should come close to the lifetime assessed in a lab scale test, provided that conditions are the same.

7 For metals this is quite well established; however, for polymers this is certainly not the case. So if the aim is to assess the lifetime of actual gears under well defined Testing conditions, this can be achieved by assessing the lifetime of tensile bars under the exact same conditions and trying to correlate the performance in terms of allowable stress for a certain number of cycles, via accurate numerical methods. There are three steps:1. Generate High-Temperature fatigue data for various Stanyl gears under oil lubrication. This approach was expected to result in fatigue failure of the gears by minimizing the amount of wear as much as possible while keep ing the temperature as constant as Generate High-Temperature fatigue data for various Stanyl tensile bars at the same temper- Incorporate the influence of tooth bending on the root stresses at various torque lev- els, and determine whether a correlation exists between fatigue lifetimes measured on gears and those measured on tensile test and MethodsStanyl is a high -heat polyamide PA 46 material made by DSM Engineering Plastics.

8 The material is character-ized by a high level of crystallinity (70%), which results in the retention of mechanical properties at temperatures Table 2 Details of gear geometriesPinionGearModule2 mm2 mmNo. of Teeth2231 Pitch Circle Diameter44 mm62 mmBase Circle mmTip Circle Diameter48 mm66 mmPressure Angle20 20 Profile Shift Tooth Width13 mm12 mmCenter Distance53 mmMaterialsteel 16 MnCr05 StanylRoot Radius modFigure 2. (Hertzian) con tact stre ss as a function of the roll angle for a steel pinion meshing w ith agear of va rious theory, the maximum is found, decreases with modulus. However, due to the change in contact path, a pre-liminary contact of the tip of the Plastic gear with the root of the pinion and a prolonged contact of the tip of the steel pinion with the root of the Plastic gear are observed.

9 This interference results in huge contact stress peaks at the beginning and end of the contact, due to the small contact radii and high forc-es, resulting in high contact stresses at high sliding velocities and, thus, in high pressure velocity values. Further study of the kinematics resulted in the expectation of substantial wear near the tip and the root of the Plastic gear, for which observations in literature were Table 1 FEA Root Stresses versus ISO 6336 values upon varying the load share by changing the stiffness of the Plastic gear in a metal pinion/ Plastic gear StressModulusISO 6336 FEAS teelE = 206 MPaPlastic GF (30%)E = 10 MPaPlastic UFE = 3 MPaPlastic UF at high TE = MPaFigure 2 (Hertzian) contact stress as a function of the roll angle for a steel pinion meshing with a gear of various March/April 2010 GEARTECHNOLOGY 61continuedabove the glass transition temperature of all polyamide materials.

10 Beyond this the material exhibits wear resistance and good fatigue properties at elevated temperatures . The following grades were incorporated in this research pro-gram: Stanyl TW341, an unfilled grade Stanyl TW200F6, a 30% glass- fiber reinforced grade Stanyl TW200B6, a 30% carbon-fiber reinforced gradeAs a comparison material, PEEK Victrex 450G (unfilled grade) was e s t t e m p e r a t u r e w a s 1 4 0 C , as lubricant Nuto H-68 oil was used (spray lubrication), a standard ESSO motor oil. All materials were subjected to a 140 C oil aging test, to ensure that no mechanical property deterioration occurred during the lifetime test geometry. The gear geom-etries are listed in Table 2; injection molding of the gears was performed by IMS Gear, Donaueschingen, Germany; tool layout was designed so that at T = 140 C, the gears were of the required size (compensating for the shrinkage and thermal expansion).